[PATCH] cgroup: Fix uninitialized variable warning

[Ross Zwisler]

Commit 1f7dd3e5a6e4 ("cgroup: fix handling of multi-destination migration
from subtree_control enabling") introduced the following compiler warning:

mm/memcontrol.c: In function âmem_cgroup_can_attachâ:
mm/memcontrol.c:4790:9: warning: âmemcgâ may be used uninitialized in this function [-Wmaybe-uninitialized]
   mc.to = memcg;
         ^
Fix this by initializing 'memcg' to NULL and then verifying that it is set
to a value before dereferencing it.

Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
---

This issue is present in v4.4-rc5 and later.

---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e234c21..69a28b3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4782,7 +4782,7 @@ static void mem_cgroup_clear_mc(void)
 static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
 {
 	struct cgroup_subsys_state *css;
-	struct mem_cgroup *memcg;
+	struct mem_cgroup *memcg = NULL;
 	struct mem_cgroup *from;
 	struct task_struct *leader, *p;
 	struct mm_struct *mm;
@@ -4805,7 +4805,7 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
 		p = leader;
 		memcg = mem_cgroup_from_css(css);
 	}
-	if (!p)
+	if (!p || !memcg)
 		return 0;
 
 	/*
-- 
2.6.3

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